Conventionally, electrical devices such as transformers, reactors, and magnetrons have employed aluminum wires, aiming at reducing their weights.
However, when an insulated electric wire is used which has a core wire made of aluminum, an oxide film is formed on the surface of aluminum of the core wire exposed to air. Hereinafter, the insulated electric wire is referred to as an aluminum wire.
When the core wire of the aluminum wire is dipped in a molten tin-based alloy, the core wire of the aluminum wire exhibits a low wettability to the molten alloy, compared to a copper wire and the like. The core wire of the aluminum wire has the following problems attributed to the wettability to the molten tin-based alloy. Hereinafter, the tin-based alloy is simply referred to as the alloy or the alloy body.
For example, the wettability to molten alloys is sometimes greatly deteriorated due to an oxide film which is formed on the surface of aluminum of the core wire. In this case, it becomes difficult to join the aluminum wire to another component such as an electric terminal.
That is, when the aluminum wire is joined to another component, unless the wettability to the molten alloy concerned is held, this poses a serious problem in joining the aluminum wire to, such as, a copper wire, another aluminum wire, or an electric terminal made of a metal material different from that of the aluminum wire.
Moreover, in the case where the aluminum wire is connected to an electric terminal or the like with an alloy, a dipping method is applied in which the aluminum wire and the electric terminal are dipped together in the molten alloy, with both being in contact with each other. However, with this method described above, aluminum is eluted to diffuse into the molten alloy, which causes the aluminum wire to become thinner in diameter.
Then, the reduction in diameter of the aluminum wire causes a problem that it becomes difficult to secure the reliability of the joining part of the aluminum wire.
Moreover, when the concentration of aluminum in the molten alloy rises, a film of an aluminum oxide is newly formed at the interface etc. where the molten alloy contacts with the atmosphere. As a result, when the aluminum wire is dipped in the molten alloy, it becomes difficult to make the aluminum wire contact with the molten alloy due to the aluminum oxide film.
Therefore, in the process of dipping the aluminum wire in the molten alloy, it is necessary to prevent joining failures and the like caused by the aluminum oxide film.
Moreover, when the connection is made using the aluminum wire, the standard electrode potential of aluminum is lower than those of other metals including copper, a copper alloy, and tin, which are used to form joining of the connection. As a result, in the contact between aluminum and a dissimilar metal, the aluminum wire poses problems of galvanic corrosion and the like attributed to the difference in standard electrode potential.
Accordingly, a joining technology is also necessary which inhibits the corrosive nature resulted from the galvanic corrosion attributed to the difference in standard electrode potential.
Moreover, aluminum used in the aluminum wire is commonly known to show a large degree of creep characteristics at low temperatures. In addition, aluminum is also commonly known to have a large linear expansion coefficient. Other than the method for connection by dipping the aluminum wire in the molten alloy, a crimping method or the like can be employed as a technique for connecting the aluminum wire to another component. For example, in the case where the aluminum wire is connected to another component by crimping, the crimped portion becomes loose with the lapse of time. The occurrence of looseness in the crimped portion causes contact resistance to rise between the aluminum wire and the component. The rise in the contact resistance, in turn, poses a problem of failure of the electrical device concerned.
Here, a case is considered where the aluminum wire is used, on at least one side, to form an electrical connection structure part which connects the wire to another component such as an electric terminal, for example. In this case, a highly-reliable joining technology has been desired, in consideration of the time-elapsed deterioration in strength and corrosion resistance, attributed to the creep property and the like of aluminum.
Hereinafter, technical trends in recent years will be briefly described, focusing on the case where the aluminum wire is used on one side to form the electrical connection structure part.
Patent Literature 1 discloses the following technology. That is, an oxide film formed on the surface of aluminum of the core wire of an aluminum wire is removed by applying ultrasonic vibrations with an ultrasonic device. Using the aluminum with the oxide film having been removed, an electrical connection structure part is formed. The aluminum wire is joined with another component.
Patent Literature 2 discloses the following technology. That is, the surface of aluminum of the core wire of an aluminum wire is activated to inhibit the formation of an oxide film thereon. Using the aluminum with the formation of the oxide film having been inhibited, an electrical connection structure part is formed. The aluminum wire is joined with another component.
Patent Literature 3 discloses the following technology. That is, an electrical connection structure part using an aluminum wire is protected with a tube or a hot melt adhesive. The protection of the electrical connection structure part allows the prevention of galvanic corrosion which would occur due to contact between the aluminum and a dissimilar metal.
Patent Literature 4 discloses the following technology. That is, as described in Patent Literature 4, a terminal is used which is provided with an insulation barrel for crimping of a covered part of an aluminum wire, a crimping barrel for continuity of the aluminum wire, and a crimping barrel for retaining the aluminum wire. The disclosed technology is such that use of the terminal allows the prevention of looseness which would occur in an electrical connection structure part that is formed by crimping the aluminum wire.
However, the technology described in Patent Literature 1 requires introduction of the ultrasonic device and the like. Accordingly, this requires the cost of introducing large scale equipment and additional man-hours, resulting in a problem.
Moreover, the removal of the film of aluminum oxide by using ultrasonic vibrations increases the possibility of a greater reduction in diameter of the aluminum wire than is desired. Therefore, a new countermeasure is necessary against the deterioration of physical strength of the aluminum wire per se. For example, in the manufacturing process, leveling-out of quality at a high level is required. Such a quality can be attained through fine control of temperature at which the alloy begins to melt, dipping conditions of the aluminum wire into the molten alloy, and the like. This entails additional costs and increased man-hours, resulting in a problem.
Moreover, in the technology described in Patent Literature 2, use of a flux causes a remarkable decrease in diameter of the aluminum wire, which requires another countermeasure against it.
In addition, some kind of technical solution is necessary to address the deteriorated wettability of the molten alloy to the aluminum wire and the galvanic corrosion attributed to the difference in standard electrode potential between the alloy and the aluminum.
Moreover, in the technology described in Patent Literature 3, in order to prevent the galvanic corrosion occurring in the joining part of the aluminum wire, the electrical connection structure part is protected from moisture by using the tube or the hot melt adhesive. This method of protection poses a problem of an increase in material cost and man-hours.
Furthermore, in the technology described in Patent Literature 4, the aluminum wire is joined by crimping with a special terminal. This method requires a special jig for the crimping. In addition, the crimp-joining of the terminal is applied for such an electrical device that is subjected to vibrations caused by a motor or the like. With the crimp-joined terminal, the aluminum exhibits a large degree of creep characteristics at low temperatures. In the joining part, time-elapsed looseness occurs. Then, the looseness in the crimp-joined joining part increases contact resistance and the like, which poses a problem of a decrease in reliability of the electrical device.
In particular, in the case of the aluminum wire being a thin wire, there has been a problem of a significant decrease in the reliability and the like.